Automatic transmissions for automotive vehicles usually include hydrokinetic torque converters and multiple ratio planetary gearing. An example of such an automatic is disclosed in U.S. Pat. No. 5,157,608, which is assigned to Ford Motor Company, assignee of this invention.
Automatic transmissions of this kind generally include a hydraulic control valve system having clutch and brake servos which control relative motion of the planetary gear elements of the transmission. The clutch and brake servos establish selectively multiple torque flow paths of differing ratios as engine torque is delivered to the vehicle traction wheels.
The hydrokinetic torque converter is located between the engine and torque input planetary gear elements of the transmission. The torque converter multiplies engine torque during vehicle acceleration and acts as a hydrokinetic coupling during steady state operation. A lock-up clutch is provided to bypass the converter during steady state operation in the highest speed ratio of a range of ratios selected by the operator, which includes an overdrive ratio. The vehicle operator may select either of two automatic shift drive ranges, drive range "3" and overdrive range "4". The disclosure of U.S. Pat. No. 5,157,608 is incorporated herein by reference to supplement the disclosure of the structure and operation of an automatic transmission and control system capable of embodying the improvements of the present invention.